6 - Hold down Shift and with the selection tool select all inner segments, highlighted with red in the pic below.
7 - While you're still holding down Shift, drag the selected rectangle's all segments to the right, so the cursor will blink blue each time you pass a grid crossing. Now you should have the outline of the wall with two windows.
8 - Click anywhere in the viewport to deselect all segments, so none of them is red. And now let's do it again: press/hold Shift and select all inner segments, basically the two windows, and drag them to the right, so when the cursor is passing over the grid points, it will comes on in blue. Now you should have 4 windows.
9 - Let's practice this a little more: click anywhere in the viewport, press/hold Shift and select all segments (the 4 windows you just drew), and this time move them down until you feel like it, to make another 4 windows. This is going to be your second floor.
you could do this by drawing single windows each and every time, but it's a lot faster this way. Try a 20 story tall building and you'll see I'm right. Once you get used to this simple routine, you can (and you WILL) use it for a lot more other things too while modeling. You will figure out how easy it is to make a building (or many other things) this way, also, how easy to edit them in seconds just the way you want it. Honestly, I seldom use the snap toggle, but at the begging it surely will help you. Please notice that you can move and change any segments, vertices, or even entire splines within the *whole* spline. Save your work, so if something goes wrong, you don't have to start all over. That's a major pain.
10 - Select any of the windows and drag it down to make a door as seen in the picture below.
11 - Select the right vertical segment of the door and move it by one grid square to the right. Repeat it with the top segment too, so you practice a little. Your door should be 4 squares wide and 5 squares tall.